Isoolefin-diolefin rubbers, for example, butyl rubber and the halobutyl rubbers derived from butyl rubber (e.g. chlorobutyl rubber and bromobutyl rubber), are used extensively in a number of applications, for example in tire manufacture. Commercially, isoolefin-diolefin rubber is typically prepared by suspension copolymerization of an isoolefin (e.g. isobutylene) with a diolefin (e.g. isoprene) in the presence of a catalyst (e.g. a Friedal-Crafts type catalyst) in a liquid non-aqueous reaction medium (e.g. comprising a diluent such as an alkyl halide) at a temperature below −90° C. in a continuous reactor to form a suspension or slurry of fine rubber “solids” in the reaction medium. The reaction temperature may be higher with the use of specialized catalysts.
A commercial apparatus comprises, among other elements, a reaction vessel in which the reaction is continuously conducted, a flash tank for evaporating liquid reaction medium and other volatile components away from the rubber which was formed in the reaction, and a discharge line for transferring rubber slurry from the reaction vessel to the flash tank. A flash tank is normally a vessel containing heated, agitated water into which the rubber is discharged, forming a crumb that is suspended in water. Steam is sparged into the flash tank to maintain the temperature and drive off volatile residual reaction medium. In conventional processes, the flash tank is normally followed by one or more stripping vessels to further remove residuals down to an acceptable level. Reaction components are normally charged to the reaction vessel in two separate feeds, a mixed feed containing monomers together with a diluent and a catalyst feed containing catalyst.
One problem encountered with such a process is the accumulation of rubber on the inner surfaces of the reaction vessel and discharge line, which can lead to fouling and considerable down time to correct the fouling. Such accumulation is a result of the “stickiness” of the isoolefin-diolefin rubbers such as butyl rubber. In order to reduce this problem, relatively high flow velocities are maintained in the reaction vessel and the discharge line. The discharge line is also steam jacketed and this is believed to lead to the formation of a vapor layer on the inside surface of the discharge line that acts like a lubricating film.
In addition, it is desirable to increase the overall energy efficiency of current commercial apparatuses. Currently, cold rubber slurry exiting the reaction vessel is transferred directly to a flash tank by the discharge line. In the flash tank, the liquid reaction medium including volatile components such as unreacted monomers, etc. is vaporized. In order to recycle the vaporized components, part of the process involves condensing those vapors and cooling the liquid back to the reaction temperature. This requires significant expenditure of energy. To reduce the amount of energy expended in cooling virgin and/or recycled reaction components, it would be desirable to capture the energy associated with the cold liquid reaction medium discharged from the reaction vessel.
WO 93/21241 to Bruzzone et al. published Oct. 28, 1993 describes a process for butyl rubber production. Polymerization of isobutylene with isoprene is conducted at a relatively high temperature of −50° C. in the presence of special catalysts. The reactor comprises a vertical cylindrical domed reaction vessel, a vertical single screw extruder (i.e. a discharge screw) associated with the reaction vessel and a horizontal double screw volatilizer connected to the discharge screw. The vertical extruder conveys and presses the solid polymer contained in the slurry upward and “squeezes” the liquid out of the slurry allowing it to drain downward (i.e. backward) toward the reaction vessel. In this way, valuable reaction medium including unreacted monomers at the reaction temperature is returned to the reaction vessel for further utilization. Residual reaction medium including residual monomers is volatilized in the horizontal screw volatilizer and fed as a gas to a heat exchanger for cooling down to polymerization temperature before re-entering the reaction vessel as a liquid.
While the process of WO 93/21241 permits recycling of some of the reaction medium by forcing it back into the reaction vessel, it suffers from several drawbacks. Firstly, the discharge screw is a single screw extruder, which is prone to fouling and is therefore unlikely to continue to operate for a worthwhile length of time. Secondly, the liquid reaction medium is forced back into the reaction vessel by the action of gravity without first being taken off stream, thereby not providing the opportunity to purify the liquid reaction medium, or the opportunity to use cold reaction medium for other cooling purposes if direct return of the reaction medium to the reaction vessel affects the polymerization process. Having no opportunity to purify the liquid reaction medium may result in a critical build-up of impurities, which would likely result in reduced catalyst efficiency and/or low molecular weight polymer being produced. The latter may cause fouling and consequently down time for cleaning the reactor. The inability to utilize the cold liquid reaction medium for other cooling purposes reduces flexibility of choice of energy recovery process. Thirdly, the use of a screw devolatilizer to volatilize residual reaction medium and monomers is not cost effective in comparison to the use of a flash tank.
GB 589,045 (the '045 patent) to Standard Oil Development Company issued on Jun. 10, 1947 describes a process for the low temperature polymerization of olefins. The '045 patent indicates that cold slurry from a reaction vessel is transported to a vibrating screen to undergo a straining or filtering operation. It further indicates that the recovered cold liquid is then recycled back into the reaction zone. Residual reaction medium and the rubber formed in the reaction are transported from the vibrating screen to a flash drum where unreacted monomers and residual reaction medium are vaporized.
In order to prevent steam from the flash tank from entering the vibrating screen and thereby contaminating the reaction medium, the '045 patent teaches a complicated system whereby a stream of sealing gas provides a positive pressure from the vibrating screen into the flash tank. Such a system is impractical because the sealing gas will mix with overhead vapors from the flash tank. The subsequent separation step would add significant cost to the process. Furthermore, the '045 patent teaches that the vibrating screen must be cooled to the reaction temperature, which would be very difficult to achieve in practice. Furthermore, it has been the experience in the butyl rubber art that the butyl rubber “solid” produced in the chemical reaction is in a fine and relatively soft particulate form that is prone to agglomeration. Furthermore, it has been the experience in the butyl rubber art that butyl rubber is “sticky” even at reaction temperatures, in contrast to statements made in the '045 patent. Therefore, one skilled in the art would expect there to be a major problem with fouling of the vibrating screen. Thus, a vibrating screen would not be expected to satisfactorily separate butyl rubber from the reaction medium. Indeed, the screening or filtering apparatus and process described in the '045 patent are not in use in any form today in the butyl rubber industry, almost 60 years after the filing of the '045 patent, an indication of the impracticality and general lack of usefulness of the technology described in the '045 patent. Finally, the '045 patent teaches that the cold rubber slurry produced in the reaction vessel contains from 1 to 10% by weight of rubber. Currently, butyl rubber reactors are typically operated to produce rubber slurries having rubber content in the region of about 25% by weight. The aforementioned problems with using a vibrating screen, particularly the fouling problem, would be exacerbated in apparatuses where the reactor produces rubber slurries having a rubber content of greater than 10% by weight.
U.S. Pat. No. 4,714,747 (the '747 patent) to Bruzzone issued on Dec. 22, 1987 describes a process for the manufacture of butyl rubber. The butyl rubber reaction itself is conducted in a self-cleaning twin screw extruder. The rubber slurry produced in the reaction extruder is transported to a vertical discharge screw which forces liquid reaction medium out of the slurry and allows it to drain back into the reaction extruder while permitting gaseous monomer-solvent mixture to vent out through a vapor outlet line at the top of the discharge screw. The rubber phase enters a heated twin screw desolventizer at the bottom of the discharge screw.
The process and apparatus described in the '747 patent has several drawbacks. Firstly, the process relies on evaporative cooling of the reaction medium to remove the heat of polymerization. It is therefore only applicable to higher temperature (i.e. −20 to +150° C.) polymerization of butyl rubber, which will only work if suitable high temperature catalysts are available. Conventional reaction temperatures are far too low for evaporative cooling to work with normal reaction media and conventional catalysts will not produce acceptable molecular weight polymer at higher temperatures. Secondly, the liquid reaction medium flows back into the reaction extruder by the action of gravity without first being taken off stream, thereby not providing the opportunity to purify the liquid reaction medium. Having no opportunity to purify the liquid reaction medium may result in a critical build-up of impurities, which would likely result in reduced catalyst efficiency and/or low molecular weight polymer being produced. Thirdly, the use of the discharge screw and a screw devolatilizer to volatilize residual reaction medium and monomers is inefficient in comparison to the use of a flash tank. Fourthly, the reaction is conducted in a twin screw extruder rather than a typical butyl rubber reaction vessel. The volume capacity of such a reaction extruder must be large in order to efficiently accommodate the reaction components. A screw extruder of such size is capital intensive thereby raising the cost of the apparatus considerably.
In the paper entitled “Extruder Isolation of Polymers and Elastomers from Latex Emulsions”, by Carl Hagberg presented at the International Latex Conference on Jul. 22, 1998, there is described a room temperature system for isolating solid polymers from latex emulsions. The system employs a counter-rotating, non-intermeshing twin screw extruder to continuously wash, de-water and dry latex particles from a latex emulsion. The system further comprises one or more mechanical filters comprising counter-rotating, fully-intermeshing twin screw extruders for removing water from the stream.
Hagberg's apparatus and process is suited for the isolation of latex particles from a latex/water emulsion at ambient temperatures, a completely different art than isoolefin-diolefin suspension polymerizations. Isoolefin-diolefin suspension polymerization is conducted at low temperature in a non-aqueous medium in conjunction with a flash tank to remove reaction medium, whereas Hagberg's latex process is conducted at much higher temperature in an aqueous medium without the use of a flash tank. Hagberg's non-intermeshing twin screw extruder design is suitable for conveying rubber in the latex system, but not for conveying rubber in the isoolefin-diolefin system, in part due to the more extreme fouling problem in isoolefin-diolefin systems.
GB 561,324 (the '324 patent) to Standard Oil and Development Company issued on May 15, 1944 describes a low temperature polymerization process for the manufacture of butyl rubber. Polymerization product is subjected to kneading as it is formed and conveyed to an extruder. Reaction medium is volatilized mainly in the kneaders. The rubber then passes to the extruder where any remaining reaction medium including residual monomers is removed as a vapor. Reaction medium is recycled as a vapor, which requires cooling before it enters the reaction vessel. This process also makes use of evaporative cooling but a low boiling component (ethylene) is added to give the desired low operating temperature.